A diplexer is a passive device that implements frequency domain multiplexing. The ports of a diplexer are frequency selective. Two ports (e.g., port M and port N) are multiplexed onto a third port (e.g., port S). The signals on ports M and N are at different frequency bands. If the different frequency bands are separated by a relatively large frequency difference, the signals on M and N do not interfere with each other. Ideally, all the signal power on port M is transferred to the port S and vice versa. All the signal power on port N is transferred to port S and vice versa. Ideally, the separation of the signals is complete.
A communication system consisting of a global positioning system (GPS) receiver and an Iridium modem connected through a diplexer has problems protecting GPS receiver from the strong Iridium transmit power. In such a configuration, whenever the Iridium modem transmits a signal, the GPS connection is lost because of the GPS receiver is saturated by the strong Iridium transmit signal. An additional filter in GPS path can minimize this effect at the cost of increased insertion loss in GPS path. The same problem occurs with a GPS receiver and an INternational MARitime SATellite (INMARSAT) modem.
FIG. 4 shows a prior art diplexer system 55, in which a GPS receiver 70 and an Iridium modem 90 are connected through a diplexer 50 to a common antenna 35. The common antenna 35 is connected at port-S of the diplexer 50. The Iridium modem 90 is connected to port-N of the diplexer 50. A GPS receiver 70 is connected to port-M of the diplexer 50. The diplexer circuit is designed using a distributed Iridium band pass filter 80 in the Iridium path and a GPS filter 60 in GPS path. The Iridium signal is reflected by GPS filter 60 and the GPS signal gets reflected by the Iridium filter 80 and, after splitting again at port S, these reflected signals are direct toward the respective desired paths. The insertion loss in a diplexer system is greatly determined by the rejection of respective filters at the operating frequency of other receiver/Modem.
The Iridium frequency is at 1616 MHz, and the GPS frequency is at 1575 MHz. This relatively close frequency spacing of only 41 MHz makes it difficult to design a filter having enough rejection at GPS frequency of 1575 MHz and at the same time having low insertion loss at Iridium frequency of 1616 MHz. Also, whenever the Iridium modem transmits, the Iridium signal saturates GPS receiver 70. The conventional diplexer system shown in FIG. 4 is unable to prevent the strong Iridium signal from saturating the GPS receiver 70.
FIG. 5 shows a simulation of the response of the prior art diplexer system 55. The plot labeled 77 is the antenna input from the antenna 35 to the Iridium modem 90 as a function of frequency. The plot labeled 78 is the antenna input from the antenna 35 to the GPS receiver 70 as a function of frequency. The insertion loss in GPS path is close to 3.2 dB and in Iridium path is close to 2 dB. The plot labeled 79 is the input from the Iridium chain to the GPS chain as a function of frequency. The isolation between the Iridium transmitter and GPS receiver path is around 20 dB.
The architecture shown in FIG. 4 is a currently implemented in prior art systems. In this diplexer system 50, when the Iridium modem 90 transmits the GPS sensitivity is reduced because of saturation of GPS receiver 70 and because the loss in GPS path is >3 dB. The increased loss in GPS path is greatly affects the GPS lock time.